It depends on where your furnaces pull return air from.
They require 35 to 85 in^2 of return duct surface area per furnace, and if they are pulling air from the basement, then the air in the coach has to go back to the basement.
Low air flow will shut down the furnace if there is a restriction of at least 25%.
2014 Fleetwood Discovery 40G -> 06 Rubicon LJ
Mechanical and Electrical Engineer
Team 065 and 0232 "De Opresso Liber"